Presentation is loading. Please wait.

Presentation is loading. Please wait.

E2 Reactions  E2 = elimination, bimolecular  Reaction is concerted (one step)  Rate = k[substrate][base] (a second order process)  E2 can occur with.

Similar presentations


Presentation on theme: "E2 Reactions  E2 = elimination, bimolecular  Reaction is concerted (one step)  Rate = k[substrate][base] (a second order process)  E2 can occur with."— Presentation transcript:

1 E2 Reactions  E2 = elimination, bimolecular  Reaction is concerted (one step)  Rate = k[substrate][base] (a second order process)  E2 can occur with S N 2.  Occurs by abstraction of H + from a C adjacent to the C with the LG.  Products follow Zaitsev’s Rule.  If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.

2 E2 Reaction Mechanism  One-step and requires a strong base.  Best transition state is anti-coplanar.  Example: t-butyl bromide + methoxide  Why don’t S N 2, S N 1, or E1 occur?

3 E2 Reaction Mechanism  Example: t-butyl bromide + methoxide

4 E2 Reaction Mechanism  Anti-periplanar alignment allows pi bond to form from sp 3 orbitals.

5 E2 Reaction Profile rate = ? k = ?

6 Factors Affecting E2 Reactions  Structure of the substrate  Strength of the base  Nature of the leaving group  The solvent in which the reaction is run.

7 Factors Affecting E2 Reactions - Structure of the Substrate  Structure of the alkyl halide: 3° > 2° >1°  This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).

8 Factors Affecting E2 Reactions - Structure of the Substrate Decide whether the following substrates could react by E1 or E2 (and by S N 1 or S N 2).

9 Factors Affecting E2 Reactions - Strength of the Base  The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?

10 Factors Affecting E2 Reactions - the Leaving Group  The LG should be good. Which of the following substrates have good LGs?

11 Factors Affecting E2 Reactions - Solvent Effects  Polarity is not so important because negative charge is spread over the transition state.

12 Zaitsev’s Rule  When two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.

13 Zaitsev’s Rule  Show the E2 elimination products that could come from:  Which E2 reaction would occur more quickly?

14 E2 Reactions - Summary  The structure of the substrate affects the rate. Relative rates for E2: 3°>2°>1°.  The base must be strong.  The LG should be good.  The solvent should be polar.  Coplanar (usually anti) transition state is required.  Products will follow Zaitsev’s Rule.  Can occur with S N 2 for 2° alkyl halides.

15 E2 Reactions - Summary  Predict the products

16 E2 Competes with S N 2 for 2° Alkyl Halides

17 E1 and E2 Reactions E1 E2 Base weak ones work strong Substrate 3°>2° 3°>2°>1° Leaving group good Solvent polar, ionizing polar, aprotic is better

18 E1 and E2 Reactions E1 E2 Rate =k[RX] =k[RX][Base] Carbocation intermediate? Y N Stereochemistry Zaitsev’s rule Rearrangement ~H, ~ CH 3 possible No rearrangements

19 How Do You Decide Which Reaction(s) Happen?  The strength and structure of the nucleophile/base is paramount. Strong ones always give a second order reaction (S N 2 or E2). If the base is bulky, E2 will occur instead of S N 2.

20 How Do You Decide Which Reaction(s) Happen?  Now, look at the structure of the substrate. 1° alkyl halides will undergo S N 2 and perhaps E2 unless rearrangement is possible. 3° alkyl halides will NOT undergo S N 2. 2° alkyl halides are the toughest to predict.

21 How Do You Decide Which Reaction(s) Happen?  Evaluate, in order, Nucleophile, strength and structure -or- Base, strength and structure Structure of the alkyl halide/carbocation Leaving group Solvent  YOU MUST BE ABLE TO DRAW THE MECHANISM!

22 Elimination Reactions  Predict the products

23 Did this product come from the reaction of 1-chlorobutane with a) sodium acetate or b) sodium t-butoxide?

24 This product came from the reaction of 1-chlorobutane with sodium acetate. The C=O and C-O peaks show ester.

25 Did this product come from the reaction of 1-chloropentane with a) sodium acetate or b) sodium t-butoxide?

26 This product came from the reaction of 1-chloropentane with sodium t-butoxide. The sp 2 C-H and C=C peaks show alkene.


Download ppt "E2 Reactions  E2 = elimination, bimolecular  Reaction is concerted (one step)  Rate = k[substrate][base] (a second order process)  E2 can occur with."

Similar presentations


Ads by Google